Electronic torque and pressure control for load sensing pumps

ABSTRACT

An electric torque and pressure control for load sensing pumps includes a variable open circuit pump with a swash plate angle sensor. The pump is connected in line with a pressure compensated load sensing control having an electrically variable pressure relief valve and orifice. Connected to the circuit is an engine speed sensor, a user input device, and a micro-controller. The micro-controller has software that controls a pressure relief setting of the electrically variable pressure relief valve in the pressure sensing control based upon signals from the swash plate sensor and the engine speed sensor and inputs from the user input device.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 14/220,201filed on Mar. 20, 2014, which claims the benefit of U.S. ProvisionalApplication No. 61/884,318 filed Sep. 30, 2013, the contents of theseapplications are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

This invention is directed toward a control for a load sensing pump. Useof a mechanical torque control is well known in the art. In knownsystems the swash plate angle is mechanically connected to a reliefvalve where the relief set point changes with the swash plate angle. Oneproblem with this system is the inability to change the torque set pointquickly for example to account for accessory loads on the engine orreduced torque at low engine speed. Another problem with known systemsis the inability to change max pressure set point on the fly.

For example, a traditional load sensing system is shown in FIG. 1. Atraditional load sensing circuit uses a variable displacement opencircuit pump with an integral control that uses a feedback pressure tomaintain a given pressure drop across a variable orifice in the system.This given pressure drop is dictated by the setting in the control atthe pump, in the example in FIG. 1 it is set to 20 bar. The pump willprovide the needed flow up to its maximum capability to try and maintaina 20 bar drop in pressure across the variable orifice. This 20 barpressure drop will be referred to as Load Sensing Margin Pressure (LSpressure).

Output pressure of the pump is equal to the required pressure to lift aload plus the drop across the variable orifice. If the pressure requiredto lift a certain load is equal to 180 bar, the resultant outputpressure of the pump would be equal to 200 bar in this example.

Input torque to the pump that must be supplied by the engine iscalculated by taking the product of the output pressure of the pump aswell as the displacement required to maintain the LS pressure dropacross the orifice. A sample of this calculation is shown below inExample 1.

As either pressure or displacement (flow) of the pump increase, theinput torque required will increase as a result. Often, when high flowsand pressures are commanded of the pump, the torque requirement placedon the prime mover exceeds the capability resulting in a stalled engine.

In addition to stalling where the input torque to the pump exceeds thetorque output capabilities of the engine driving, the result is operatorfrustration and/or poor performance. Systems with dual set-points areknown but are very complex and expensive. Therefore, a need exists inthe art for a system that addresses these deficiencies.

An objective of the present invention is to provide a control for a loadsensing pump that can change a torque setting quickly.

Another objective of the present invention is to provide a control for aload sensing pump where a maximum pressure set point can be changed onthe fly.

A still further objective of the present invention is to provide acontrol for a load sensing pump that reduces the possibility of theengine stalling.

These and other objectives will be apparent to one of ordinary skill inthe art based upon the following written description, drawings, andclaims.

SUMMARY OF THE INVENTION

An electric torque and pressure control for load sensing pumps includesa variable open circuit pump with a swash plate angle sensor. The pumpis connected in line with a pressure compensated load sensing controlhaving an electrically variable pressure relief valve and orifice.Connected to the circuit is an engine speed sensor, a user input device,and a micro-controller. The micro-controller has software that controlsa pressure relief setting of the electrically variable pressure reliefvalve in the pressure sensing control based upon signals from the swashplate sensor and the engine speed sensor and inputs from the user inputdevice.

In some embodiments, the software continuously calculates a maximumpressure based on signals received from the swash plate sensor regardingthe angle of the swash plate. The calculated maximum pressure is equalto the torque level the engine can produce at that pressure withoutstalling. The software, via the micro-controller, sends a current to theelectrically variable pressure relief valve to produce the calculatedmaximum pressure. In this way, the engine is able to maintain a torquelevel required by an operator's command that is below or equal to amaximum torque the engine can provide without stalling (i.e., torquecapacity) by relieving pressure to reach the calculated maximumpressure. The production of the calculated maximum pressure isaccomplished without changing the operator's command, which in turnprevents the inefficiencies and stalls associated with the prior art.

As set forth, a calculation and a command are distinct operations of thepresent invention. A command, such as one initiated by an operator, isan instruction that the software receives as an input and accomplishesthe associated output. Another example of a command is disclosed by U.S.Ser. No. 10/503,726 to Lonn, which indicates that a position of athrottle (pedal) is a command by an operator that is sensed and sent toa control unit as an input and the control unit sends the correspondingspeed output to regulate a motor. In contrast, a calculation requires acomputational determination be completed based on an input in order toreach a result.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a prior art load sensing system;

FIG. 2 is a schematic view of an electronic torque/pressure controlcircuit;

FIG. 3 is a chart comparing pump displacement with maximum torquepressure;

FIG. 4 is a chart comparing pump displacement with current to valve;

FIG. 5 is a chart comparing pump displacement with pressure;

FIG. 6 is a chart comparing pump displacement with system displacement;

FIG. 7 is a schematic view of an electronic torque/pressure controlcircuit;

FIG. 8 is a schematic view of a torque control circuit with load holdingvalves;

FIG. 9 is a schematic view of a torque control circuit with a pressurecompensated pump;

FIG. 10 is a chart showing a margin allocation in torque control bycomparing displacement with pressure; and

FIG. 11 is a chart showing a margin allocation in torque control bycomparing displacement with pressure.

DETAILED DESCRIPTION OF THE INVENTION

The system 10 is comprised of a variable open circuit pump 12 with aswash plate angle sensor 14. The pump 12 has a pressure compensated,load sensing control 16 with an electrically variable pressure reliefvalve 18 and orifice 19 built into the input side of the control 16. Anexternal micro-controller 20 and software 22 utilize the signal from theswash plate angle sensor 14, as well as engine speed and userprogrammable inputs to control the pressure relief setting of the valve18 in the control 16.

The Electronic Torque/Pressure Control Circuit 24 (ETL) is created bythe addition of the items shown in FIG. 2 to a conventional load sensingcircuit. The additional items include:

Micro-controller 20 and software 22

Electronically proportional pressure relief valve 18 default to max.

Orifice 19 at LS input of the pump control 16

Swash plate angle sensor 14

Engine speed sensor 26

User input device 28

Basic ETL Circuit Operation

Oftentimes with load sensing open circuit systems, the torque requestedto be supplied by the engine exceeds the engine's capabilities. Whenthis happens, the operator is required to reduce his commands, slowingthe machine which can make it difficult to operate efficiently.Alternatively, the engine simply stalls requiring the operator torestart the machine.

Starting with the engine torque calculation in example 1.

Assume the operator of that machine were commanding this operation, andthen encountered some resistance to the circuit that raised the force onthe cylinder, and the resultant pressure in the circuit to 300 bar (320bar at the pump). With no change in the valve command, the pump will tryand maintain the same output flow at the new higher pressure. Theresulting new torque requirement to the engine is shown in Example 2.

If the engine on the machine is only capable of 150 Nm of output torque,this new load and sustained flow command would overwhelm the engine andresult in a stalled condition if the operator continued the command.With basic ETL, the system 10 can control the stroke of the pump 12 byregulating the LS pressure in the control 16, in turn maintaining atorque level at or below the maximum torque that the engine can provideand keeping the engine from stalling.

As shown in FIG. 3, as an example there is a large area in which thepump 12 is capable of operating in, that would result in an engine stallcondition. The line 30 marked by triangles shows the maximum torquelevel that the engine is capable of delivering to the pump 12. The line32 marked by squares shows the constant maximum pressure limit usuallyemployed with a traditional load sense system.

During machine operation, the software 22 is continually monitoring theangle of the swash plate in the pump 12. The software 22 uses the swashplate angle to calculate a maximum pressure that would result in atorque level that the engine could produce at the given displacement,and sends the correct current to the proportional pressure relievingvalve 18 in the pump control 16 to achieve that maximum pressure. Shownin FIG. 4, as swash plate angle increases, the current to the pressurerelief valve 18 increases (decreasing its setting) limiting the amountof torque the pump 12 can absorb.

Using this control logic, electronic torque limiting is able to clip offthe area 34 in FIG. 3 that results in engine stalling, and insteadallows the hydraulic system 10 to always deliver maximum possiblepressure for a given displacement without engine stalling.

Revisiting the example once again, this time with ETL active:

-   -   1. The operator commands a flow and displacement equal to our        first example: 45 cc's and 200 bar.    -   2. The machine encounters a load which raises system pressure to        320 bar.    -   3. ETL is constantly active, and the pump 12 quickly destrokes        to an angle that will allow the load to be lifted without        stalling the engine.        ETL Operation from a Mechanical Standpoint    -   1. The operator commands a flow and displacement equal to our        first example: 45 cc's and 200 bar.    -   2. The machine encounters a load which raises load pressure to        300 bar (320 bar seen at pump).    -   3. The operator maintains the same command. 300 bar load        pressure is transferred down the LS line to the electronically        proportional pressure relief valve 16. 320 bar pressure is        transferred through the variable orifice 19 to the pump 12 and        to the pump controls 16.    -   4. The LS pressure is relieved at a setting calculated by the        micro controller 20 based on the angle of the swash plate. This        lowers the pressure on the LS side of the pump control 16.    -   5. High pressure on the pump side of the pump control 16 shifts        the control to port oil to the servo piston, de-stroking the        pump.    -   6. As the pump 12 de-strokes, the software 22 is reducing        current command to the LS variable relief valve 18, allowing LS        pressure on the pump control 16 to increase.    -   7. The pump 12 will continue to de-stroke and the LS pressure        will continue to increase based on swash plate angle until a 20        bar delta between pump output and LS pressure is reached.        Torque Control with Load Holding Valves

A system comprised of a traditional mechanical torque control withmultiple functions and a load holding or load drop check valve canencounter conditions when the pump outlet pressure is limited below apressure that can lift the “checked” load, and when that function isenabled, it is unable to move. The use of electronic torque controlalong with electronically controlled valves, a pressure transducer, anda software solution can alleviate this problem.

In FIG. 8, for example, the valve 18 for function 1 is opened anddemands a pressure of 150 bar to lift the load and a flow that togetherwill exceed the current torque limit setting of the ETL software 22. Inthis scenario, the ETL will be regulating the displacement of the pump12. If the valve 18 for function 2 is opened, which requires a pressureof 250 bar to lift the load, the check valve 36 will continue to supportthe load, and the required pressure will not be communicated back to thepump control 16 to allow ETL to function properly and lift the load. Tosolve this problem, a pressure transducer 38 is added to monitor thepressure required to lift function 2 when it is commanded by theoperator. When a command is issued for function 2, but the currenttorque set point of the pump 12 does not allow the load to be lifted,the software 22 will pull back the command of function 1 (or multipleother functions) until the pump displacement is decreased to a pointthat will allow a high enough pressure to lift the load on function 2.In considering this function, one must remember that the ETL software 22continuously monitors swash plate angle and will increase the pressurelimit of the pump 12 as pump displacement decreases so as to maintain anacceptable torque level to the engine.

Torque Control On Pressure Compensated Pumps

In backhoe systems it is common to use a pressure compensated pump withtorque limiting pump control and a manually operated open center valvestack. All the advantages previously listed in the load sensing circuitstill apply to the pressure compensated system. Additionally, as shownin FIG. 9, it is common to have a special dump valve 40 to reduce theset point of the PC pump during engine cranking (primarily in coldconditions). The issue is that when the oil is cold, there is asubstantial amount of pressure required to push the oil through the opencenter valve 42. Without any additional components the torque limitingsystem can reduce the pressure set point of the PC during cranking toreduce outlet pressure and displacement, thus reducing the load on theengine's starter.

Torque Control and Margin Erosion Across Valves

In proportional valve groups, especially compensated valves, the designof the valves usually requires a minimum pressure drop across the valve(or margin) for it to operate properly, and properly communicate theload sense pressure back to the pump. As discussed previously, torquecontrol functions by shifting the margin across the valve to an orifice19 located in the pump control 16. As torque control further reducestorque, the margin across the valve 18 can drop to levels where it maynot function correctly. This can be especially noticed during low engineRPM operation where the level of torque reduction is quite high.

FIG. 10 outlines the pump outlet pressure (Ppump), the actual loadpressure (PLS) which is the pressure actually working on the load, andthe pressure seen at the load sense control of the pump (Pctrl) which isafter the relief valve 18 and orifice 19.

A starting condition shown by the X at the end of the arrow requires adisplacement of I47 cc to maintain the margin across the valve 18 and apressure of 75 bar to lift the load. At this condition, the point is notunder influence of the torque control, and the entire margin issatisfied by the drop across the proportional control valve 44. If thecommand to the valve 44 remains the same, as the load pressureincreases, it will first travel upward until the PLS line tums to theleft. It is at this point that torque control is starting to becomeactive and relieve pressure at the control. As the pressure continues toincrease (following the PLS line), the pump 12 continues to destrokewhich will reduce the flow through the control valve 44. As previouslystated this valve is still receiving the same command, so the reductionin flow lowers the pressure drop across this valve 44. The totalpressure drop between the pump outlet (Ppump) and (Pctrl) is still beingsatisfied by the increasing pressure drop across the orifice 19 in theLS control 16, thereby satisfying the required margin to keep the pump12 from going into stroke. As the pressure continues to rise, one cansee that the pressure drop to satisfy the margin requirement of the pump12 continues to shift away from the control valve 44 and to the orifice19 at the LS control 16 on the pump 12. The point at which it reachesthe vertical line is the point at which the margin across the controlvalve 44 has dropped to a point where it may no longer functioncorrectly. It is at this point machine performance may begin to suffer,and further pump angle reduction can cause poorer valve performance.

To solve this problem, a method of controlling the total valve flowrequest has been utilized. The employed algorithm seeks to limit thevalve opening so that the torque limiter is not impacted by marginerosion while avoiding unnecessarily limiting the valve output when thetorque limiter is not actively regulating. By using electronicallycontrolled valves in conjunction with the pump angle sensor and amicrocontroller, it is possible to manipulate the shift of the marginfrom the control valves 44 to the orifice 19 in turn, allowing furtherdestroking the pump to meet load and output torque requirements.

Looking once again at FIG. 10, we can take a closer look at the verticalline in the graph which represents the minimum margin requirement forproper control valve function (let's assume 7 bar for this example).That means the difference between the middle curve (PLS) and the uppercurve (Ppump) is 7 bar at the intersections of the vertical line. If theload pressure were to continue under the steady valve command in thisexample, the standard torque control would continue to destroke the pumpto the left of this line and control valve performance would start todeteriorate. The creation of these performance lines are based on theinitial conditions of the valve, load, and pump. If we were to changethe opening of the control valve (flow request) it is possible to changethe nature of these curves, and allow the pump to further destrokewithout further margin erosion. Continuing the example, if the requestfrom the pump is lowered from the full I47 cc to 115 cc, thecharacteristics of the PLS curve are re-shaped, and in turn changes theshift of margin discussed above. The now slightly more restrictive valveopening increases the relative margin across itself, allowing forfurther pump destroking meeting the increased load demands. As you cansee in FIG. 11, reducing the valve request from 147 cc to II5 cc forthis example allows full system pressure to be reached before the marginerosion across the valve becomes an issue.

What is claimed is:
 1. A control system for a load sensing circuit,comprising: a variable open circuit pump with a swash plate anglesensor; a pressure compensated load sensing control having anelectrically variable pressure relief valve and orifice; an enginehaving a maximum torque output capability delivered to the variable opencircuit pump; an engine speed sensor; a user input device; amicro-controller having software that controls a pressure relief settingof the electrically variable pressure relief valve in the pressurecompensated load sensing control; and wherein the software continuouslycalculates a maximum pressure that would result in a torque leveldelivered by the engine to the variable open circuit pump at a senseddisplacement of a swashplate and sends a current to the electricallyvariable pressure relief valve to produce the calculated maximumpressure.
 2. The system of claim 1 wherein the software calculates themaximum pressure required that would result in an operative torque levelproduced at variable swash plate displacements.
 3. The system of claim 1further comprising a pressured transducer that monitors a pressurerequired for a lift function.
 4. The system of claim 1 furthercomprising a dump valve to reduce a set point of a pump engine cranking.5. The system of claim 1 wherein the software adjusts a first functionoutput by a first command when a second command is received to output asecond function and a torque set point of the pump does not allow a loadto be lifted until pump displacement is decreased to a point that willhave a high enough pressure to lift the load on the second function. 6.The system of claim 2 wherein the operative torque level is where andwhen an input torque to the pump does not exceed a torque outputcapability of the engine and does not result in a stalled engine.
 7. Thesystem of claim 1 wherein a control for the load sensing pump changes amaximum pressure point automatically without manual intervention.
 8. Acontrol system for a load sensing circuit, comprising: a variable opencircuit pump with a swash plate angle sensor; a pressure compensatedload sensing control having an electrically variable pressure reliefvalve and orifice; an engine having a maximum torque capabilitydelivered to the variable open circuit pump; an engine speed sensor; auser input device; a micro-controller having software that controls apressure relief setting of the electrically variable pressure reliefvalve in the pressure compensated load sensing control; and the softwareis configured to calculate a maximum pressure based on signals receivedfrom the swash plate angle sensor, wherein the maximum pressure is equalto a maximum torque level the engine can produce without stalling at apressure; the software is configured to send a current to theelectrically variable pressure relief valve to produce the maximumpressure while maintaining a torque level required by an operator'scommand that is no higher than the maximum torque level.
 9. The systemof claim 8 wherein the software is configured to send the current to theelectrically variable pressure relief valve while the operator's commandis maintained.
 10. The system of claim 8 wherein the software isconfigured to send a current to the electrically variable pressurerelief valve to achieve the calculated maximum pressure.